Solve a Slitherlink puzzle by drawing its single closed loop.
Slitherlink, also called Loop the Loop or Fences, is drawn on a grid of dots.
The goal is a single closed loop along the edges between dots so that every
numbered cell has exactly that many of its four sides used by the loop. Type
the clues into the cells, leaving a cell blank where there is no clue, then
solve.
Enter 0 to 3, or leave a cell blank.
Solution
How it works
The solver decides each edge on or off in turn. It keeps every dot at a
degree of 0 or 2 so the lines can only form loops, keeps each clue on track
for its exact count, and uses a connection tracker so that the moment an
edge would close a loop it is accepted only if that loop already satisfies
every clue. That stops the search from ever building a second, separate
loop.
A proper Slitherlink has a single loop; the solver reports when more than
one loop fits.